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Questions and Answers
What does an electrocardiogram (ECG) primarily measure?
What does the ECG provide information about?
During which phase of the cardiac cycle do the AV valves remain open?
Which of the following is a reason to perform an ECG?
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What cardiac event does an ECG NOT assess?
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What is the primary indication for using a resting ECG?
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What is the role of Bachmann’s bundle in the heart's conduction system?
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What happens during the passive filling phase of the ventricles?
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What occurs during diastole when the AV valves are opened?
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What marks the beginning of ventricular systole?
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During the period of ejection in systole, which valves are closed?
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What happens during the isovolumic relaxation phase of diastole?
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What is the primary function of the four extremity electrodes in an ECG?
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How does blood flow into the ventricles during passive ventricular filling?
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What should be maintained when placing ECG electrodes on the extremities?
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Which event follows the closure of the AV valves during the cardiac cycle?
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Where is electrode V1 placed?
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What does the 3 channel ECG primarily monitor?
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Which lead provides information from the left arm to the left leg?
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What happens when the SA node or its pathways are damaged?
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In a junctional rhythm, the heart is paced by which structure?
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What characterizes second-degree heart block on an ECG?
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Which electrode is placed under the breast in women according to official recommendations?
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What does damage to the AV node result in?
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Study Notes
ECG Basics
- An ECG represents the sum of many cardiomyocyte action potentials.
Action Potential: Myocyte
- No information is given about the action potential of myocytes
Conducting System of Heart
- Contains Bachmann's bundle, internodal fibers, and the AV node
The different ECG waves
- No information is given about what the different ECG waves represent
Depolarization and Repolarization at the Ventricles
- No information is given about depolarization and repolarization
###Â ECG for Heart Health
- An ECG can record the electrical activity of your heart, and shows information such as heart rate, rhythm, and the size of the heart.
- An ECG can be used to identify whether there is evidence of enlargement of the heart due to hypertension, or a heart attack.
- There is a difference between the electrical activity of the heart and the mechanical activity of the heart.
Ventricular diastole: Period of Passive filling
- Ventricles are relaxed, AV valves open, and blood flows into the ventricles.
- This accounts for most of the ventricular filling.
Ventricular diastole: Period of Active filling
- Atria contract, completing ventricular filling.
Ventricular systole: Period of Isovolumic contraction
- Ventricular contraction causes the closure of the AV valves, beginning systole, and the semilunar valves remain closed.
Ventricular systole: Period of ejection
- Continued contraction of ventricles ejects blood into the ventricles, causing the semilunar valves to open.
Ventricular diastole: Period of Isovolumic Relaxation
- Blood flows back toward the relaxing ventricles, causing the semilunar valve closure.
ECG Electrodes
- Ten electrodes are used:
- Four on the extremities: LA (Left Arm), RA (Right Arm), N (Neutral, on the Right Leg), and F (Foot, on the Left Leg).
- Six on the chest: V1 (4th intercostal space, Right of Sternum), V2 (4th intercostal space, Left of Sternum), V3 (Between V2 and V4), V4 (5th intercostal space in the nipple line), V5 (Between V4 and V6), and V6 (Midaxillary line at the same height as V4).
- There is no difference in function if the electrodes are placed proximal or distal on the extremities, as long as they are consistent.
The Bipolar Leads
- The 3 channel ECG uses 3 or 4 electrodes to monitor heart rhythm.
- These leads are not sufficient to identify ST elevation as no lead provides information about the anterior wall.
- If ST changes are seen during 3 or 4 channel monitoring, a 12 lead ECG needs to be obtained.
- The extremity leads are:
- Lead I: Right arm to Left Arm.
- Lead II: Right arm to Left Leg.
- Lead III: Left arm to Left Leg.
###Â Some Heart Rhythm Problems
- Conduction deficits can cause disturbances to the heart's rhythm.
- If the SA node or intermodal pathways are damaged, the AV node takes over.
- Heart beats at a slower rate.
- Heart block occurs when the AV node is damaged and partially or totally releases the ventricles from the influence of the SA node.
- Ventricles then begin to beat at their own rate, which is slower than normal.
ECG Examples
- Normal sinus rhythm: Normal heart activity
- Junctional rhythm: SA node does not function, P waves are absent. Heart is paced by the AV node at 40-60 beats per minute.
- Second-degree heart block: Some P waves are not conducted through the AV node. More P waves than QRS waves are seen.
- Ventricular fibrillation: Chaotic and grossly irregular ECG deflections.
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